The present invention relates generally to a semiconductor memory device and, more particularly, to a semiconductor memory device with dynamic memory cells and a manufacturing method thereof.
A conventional dynamic random access memory having a plurality of dynamic memory cells each of which comprises a transistor element and a capacitor element has been widely used in the field of dynamic semiconductor memory devices. The dynamic memory cells have word lines and bit lines which are connected in a matrix form.
In general, the capacitor included in each of the memory cells comprises a MOS capacitor. In this case, the thickness of a gate insulating film of the capacitor is about 300 to 500 .ANG. in a 64K-bit dynamic RAM (dRAM), about 200 .ANG. in a 256K-bit dRAM, and about 100 .ANG. or less in a 1M-bit dRAM. In a memory having such a thin insulating film, the frequency of film-isolation breakage is one of the most important factors in determining the yield of the device.
An insufficient breakdown voltage of the insulating film of the MOS capacitor may result from the fact that impurities are introduced during the process of forming the insulating film, or that the insulating film itself is defective. In addition to these reasons, electrostatic breakdown occurring during the manufacturing process is an important reason for an insufficient dielectric breakdown voltage. In particular, when reactive ion etching (RIE) using free charge carriers and sputtering etching are performed for micropatterning, electrostatic breakdown of the insulating film during the etching process is a very important factor in the above drawback. Furthermore, electrostatic breakdown may also occur from a noise component mixed in a power source line during the operation of the circuit. The above problem is serious and typically occurs when the gate insulating film is 100 to 200 .ANG. or less.